I want to believe this, but it's hard for me to take this at face value. It's been about 4 years since I've run pmOS, so my experience IS very out of date, but I also have a hard time believing that the experience has radically changed in the meantime.
The short is that yes the GNOME/KDE apps do often look more impressive, but they suffer the same sort of malaise which seems to have infected Linux desktops sometime since Eternal September, and between the sporadic crashing and "this doesn't feel right", it's really hard for me to accept "It's more polished than AOSP!".
pmOS's installation page opening with a warning:
Make sure you read state of postmarketOS before installing postmarketOS.
The goal is to make postmarketOS usable for everyone, but we are not there yet. Usability and most importantly stability issues need to be worked out first. If you are looking for an OS that is as usable as iOS or Android, this project is currently not for you.
GrapheneOS provides drastically better usability, robustness, overall functional and app compatibility. Privacy and security are also drastically better in AOSP and especially GrapheneOS than that desktop Linux software stack ported to mobile.
All of the default apps in GrapheneOS are being rapidly overhauled or replaced. It wasn't a priority due to the incredibly good open source app ecosystem with many existing alternatives available. There isn't a similarly large and high quality open source mobile app ecosystem available for what's being promoted.